Blends which are polar have been made by dynamic vulcanization using a condensation reaction with the evolution of a small molecule, but such a reaction is relatively slow, compared to the addition dynamic vulcanization of an olefinic rubber and a polyolefin thermoplastic. The condensation reaction cannot be carried out in the presence of enough plasticizer to facilitate blending the components at a temperature below which the physical properties of the blend will be affected deleteriously. If the evolved small molecule is not disposed of, it becomes trapped in the blend resulting in a spongy mass. Therefore such blends are preferably made in a substantially continuous process with adequate venting, because it is impractical, adequately to seal a batch mixer such as a Banbury, for staged addition of components and removal of products of reaction, while it operates at the required temperature in the range from about 200.degree. C. to 275.degree. C.; and it is generally too costly to prepare a blend in separate and distinct stages using an elongated mixing-reaction zone which is relatively short, and pelletizing the dynamically vulcanized blend at each stage before introducing the pellets to the next stage.
Though a barrel of a continuous mixer or mixer-extruder is available in different lengths, commercially available barrels have a length (L) to diameter (D) ratio no greater than 80; they are typically in the range from about 20 to 80. Therefore, for all practical purposes, a continuous process for making the blend must be completed within a barrel having a L/D no more than 80; and, if the barrel has a L/D closer to 20, the process may have to be carried out in two or more separate stages, provided means for removing small molecules evolved due to successive condensation reactions, are provided.
Processes for making blends with non-polyolefinic "plastics" are taught in U.S. Pat. No. 5,589,544 to Horrion and in copending patent applications Ser. Nos. 08/686,782 now U.S. Pat. Nos. 5,783,631 and 08/686,798 now U.S. Pat. No. 5,777,033 filed Jul. 26, 1996. By "non-polyolefinic" plastic is meant that the plastic is substantially free of polyolefin chains. The term "plastic" refers to a resin selected from the group consisting of polyamides, polycarbonates, polyesters, polysulfones, polylactones, polyacetals, acrylonitrile-butadiene-styrene (ABS), polyphenylene oxide (PPO), polyphenylene sulfide (PPS), styrene-acrylonitrile (SAN), polyimides, styrene-maleic anhydride (SMA) and aromatic polyketones, any of which may be used by itself or in combination with another. Most preferred engineering thermoplastic resins are polyamides and polyesters.
In the Horrion '544 patent, in the first stage, a cured rubber concentrate (CRC) is prepared which, in a second stage, is subsequently blended with an engineering thermoplastic, optionally with a compatibilizer. The CRC is formed from a mixture of a curable elastomeric acrylic copolymer ("rubber") and an acrylic "polymeric carrier" which does not react with a curative for the rubber; and the rubber is immiscible with the polymeric carrier.
Referring to FIG. 1, there is schematically illustrated the mixing (vented) barrel of an extruder in which the two stages of the '544 process are continuously carried out. Into the first stage of the extruder is fed a mixture of curable rubber 1 and curable rubber 2 (referred to as the "carrier"), along with a curative for curable rubber 1, optionally with additives which may include a compatibilizer, and the mixture dynamically vulcanized ('544 text bridging col 7 line 51 and col 8 line 11). The CRC (Blend A) is formed because only curable rubber 1 is cured. In the second stage, the CRC is blended with plastic and additional additives and compatibilizer, if necessary (col 8, lines 57-67); the carrier may be crosslinked while mixing with the plastic using a curative different from the one used for curing the curable rubber (cot 9, lines 1-5). However, the plastic blend (Blend H) formed directly in the second stage, as described by Horrion, is susceptible to porosity. Porous (spongy) pellets of Blend H trap water in the pelletization step, and are difficult to handle in subsequent processing steps. It is impractical to dry such wet pellets prior to subsequent processing. The pellets trap water which results in pellets unsuitable for injection molding, when the blend is made in the desired plastic to rubber ratio, using an extruder, to make relatively soft TPVs.
The '798 application teaches a process in which a first curable acrylic rubber and a curable terpolymer are vulcanized in the presence of a curing agent and plastic, to form a blend which has a single low temperature brittle (LTB) point. The '782 application teaches a process in which at least first and second acrylic curable rubbers, each with a different functional group, are cross-linked in plastic, substantially without benefit of a curing agent. The curable rubbers are compatible with each other and also with the engineering plastic. By "compatible" is meant that the rubbers form a mixture in which a second phase can co-exist with the continuous phase without the use of a compatibilizer or a surface active agent. Typical acrylic rubbers have an C.sub.1 -C.sub.10 alkyl group in combination with one or more groups chosen from C.sub.2 -C.sub.3 olefin, carboxyl, hydroxyl, epoxy, halogen, and the like.
No known process utilizes a three-stage process including sequential first and second condensation reactions, the first to form an intermediate hard blend with a first curable acrylic rubber, and the second to form a second cured blend of another curable acrylic rubber within the intermediate hard blend, substantially without using a plasticizer, so as to result in a final blend having a hardness less than 30 Shore D, preferably less than 90 Shore A. The first curable rubber contains a different functional group from that of the second curable rubber.
As seen by the comparative illustrative example provided herebelow, a blend made by the two-step process taught by the Horrion '544 patent has surprisingly different properties, though the '544 blend has the same components present in the same relative amounts as in a blend made by the three-step process of this invention.